Method and device for the spectral analysis of light

ABSTRACT

The method uses a physical phenomenon of dispersion of the optical rotation for identification of the spectral characteristics of light. Polychromatic linearly polarized radiation passes through the environment that rotates a polarization plane of its spectral components, depending on their wavelength. After a subsequent passage through the analyzing polarizer, a dependence of the light intensity S((φ) on the angle φ, that the analyzing polarizer forms with the polarization plane of the analyzed light, is measured. S((φ) is in a mathematical relationship with the spectrum of the analyzed radiation I(λ), where λ is a wavelength. S((φ) allows for the determination of the spectral characteristics of the analyzed radiation.  
     In devices based on the above principle, the collimated polarized beam of the analyzed radiation passes first through the optical element that exhibits a dispersion of the optical rotation, i.e. rotator ( 4 ), then through the analyzing polarizer ( 5 ), and after a projection is detected by a proper detector ( 7 ). The detector measures S((φ) as a function of the angle φ of the analyzer. From S(φ) the parameters of the spectrum I(λ) are determined.

TECHNICAL FIELD

[0001] The invention pertains to the spectral analysis of light.

STATUS OF THE CURRENT TECHNOLOGY

[0002] The currently used procedures for the spectral analysis of lightcan be divided into three groups. The first and oldest approach is basedon the angular spectral separation of light. The light, upon interactionwith a suitable optical element (optical prism, optical grating),changes the direction of propagation. This direction depends on thewavelength. Such spectrally separated components can be independentlyanalyzed. The second, frequently used procedure is based on thephenomenon of light interference when a spectrum of the analyzedradiation can be calculated from the interferrogram. The third, recentlypublished procedure uses the phenomenon of the dispersion of the opticalrotation (CZ Patent 284 282). When the linearly polarized light passesthrough a proper element (rotator), its polarization plane rotates as afunction of wavelength. This can be used for identification of itsspectrum.

THE DISCLOSURE OF THE INVENTION

[0003] The subject of the invention is a new method for identificationof spectral characteristics of light. The method is based on thephenomenon of dispersion of the optical rotation. The invention is amodification of the method patented in CZ 284 282. The principle of themethod for identification of the spectral characteristics of light isthat the analyzed beam of radiation is converted to a linearly polarizedparallel beam of radiation and this collimated polarized radiation issubjected to the optical rotation in the environment with a dispersionof the optical rotation (rotator). The extent of this rotation isselected according to the interval of the wavelengths in which thespectral analysis is being done. After passing through the rotator, thepolarization plane of the individual spectral components is rotated andthe extent of the rotation depends on the wavelength. Such parallel beampasses through the second polarizer. The polarization plane of thispolarizer forms an angle φ=φ₁ with the polarization plane of theanalyzed radiation. Then, the intensity of the output beam is measured.The procedure is repeated for the fixed parameters of the rotator anddifferent values of the angle φ from the interval (φ₁,φ₂). Suchprocedure yields functional dependence of the output intensity S(φ) onthe angle φ. Spectral characteristics of the analyzed radiation are thenextracted from this dependence by a mathematical analysis. Thecharacteristics can be either a full spectrum, or some characteristicspectral parameter only, for example a location of the spectral maximum,a spectral half-width, etc.

[0004] During the measurement of the spectral characteristics of a pointlight source, the collimated polarized radiation passes through theactive environment that exhibits the same dispersion of the opticalrotation in the entire crossection of the analyzed beam. The outputlight intensity is measured with a single-channel detector.

[0005] During the measurement of the spectral characteristics of aplanar light source, the collimated polarized radiation passes throughthe active environment that exhibits the same dispersion of the opticalrotation in the entire crossection of the beam and the output intensityof light is measured with a multi-channel detector.

[0006] The device according to the presented method consists of acollimating optical system and a detector. Importantly, the firstpolarizer, rotator, and the second polarizer are placed between thisoptical system and the detector. The rotator must exhibit a non-zerodispersion of the optical rotation and its parameters do not changeduring the measurement. When collimated or polarized radiation isanalyzed, the device does not contain collimating optical system or thefirst polarizer, respectively.

[0007] An advantage of the method for the spectral analysis of light andan advantage of the device using the new method is that in comparisonwith the previously published method, the parameters of the rotator donot change during the measurement. Only the second polarizer rotatesinstead.

[0008] For an identification of spectral characteristics of the inputradiation according to the new method, the output intensity is measuredfor different positions of the analyzing polarizer. For collimated,polarized, polychromatic beam of light from the spectral interval(φ₁,φ₂), the intensity S((φ) after passing through the rotator and theanalyzer is measured as a function of the angle φ from the interval(φ₁,φ₂): S(ϕ) = ∫_(λ₁)^(λ₂)I(λ)cos²[θ(λ) − ϕ]λ

[0009] where I(λ) is a spectrum, φ is an angle between the polarizationplane of the analyzer and a polarization plane of the linearly polarizedinput light, and θ(λ) is an rotation angle of the polarization lane forthe radiation with wavelength λ after a passage through the rotator.Since the function S((φ) has a period π, measurements for angles φ₂-φ₁>π do not provide any new information. Characteristics of the spectrumI(λ) are determined from the dependence of S((φ) on φ, according to theformula shown above. Depending on the complexity of the function I(λ),it can be either the full spectrum, or only some of its spectralparameters, e.g. wavelength of the spectral maximum, spectralhalf-width, etc.

[0010] In the device that works according to the above presentedprinciple, the linearly polarized parallel beam of radiation firstpasses through the optical rotator where the polarization plane of theindividual spectral components of radiation rotates in dependence ontheir wavelengths, further passes through the analyzer, and then itimpinges on the detector which measures S((φ) as a function of therotation angle of the analyzing polarizer. Characteristics of thespectrum I(λ) are then determined from the formula for S(φ), aspresented above.

[0011] Illustration

[0012] The principle of the method for the spectral analysis of theelectromagnetic radiation and the device for its utilization are shownin FIG. 1.

EXAMPLES Example No. 1

[0013] A multi-channel measurement of the spectral characteristics ofthe planar light source—FIG. 1. The light emerging from the aperture 1is collimated by the optical system 2, and linearly polarized by thepolarizer 3. The light then passes through the rotator 4 analyzer 5, andit is projected by the optical system 6, onto the detector 7, whichmeasures the function S(φ). Different elements of the light source areprojected on different pixels of the multichannel detector whichsimultaneously measures corresponding functions S((φ). A planar spectralmap of the investigated object is obtained by their analysis. It can beeither a spectrum, or some of its parameters that is sufficient for thespectral characterization of the analyzed planar source of light.

Example No. 2

[0014] This method allows for a simple determination of wavelength of amonochromatic radiation. For a monochromatic radiation with thewavelength λ₀, and the angle φ_(λ0) of the analyzer such thatθ(λ₀)−φ_(λ0)=(2k−1)*π/2, where k is a primary number, applies that theS((φ)=0. It means that there exists a specific rotation angle of theanalyzer for each given wavelength, for which the passing intensityequals zero. Because the rotator exhibits non-zero dispersion of theoptical rotation, different rotation angles for different wavelengthsare required to make output intensity equal to zero. With a properlyselected rotator, it is possible to calibrate the angular scale of theanalyzer absolutely in wavelengths. Because this method of the “crossedpolarizers” is very accurate, such absolute calibration is also veryaccurate. This procedure can be used for identification of thewavelength of the light, for example from tunable dye lasers.

[0015] The device for the absolute spectral calibration of themonochromatic non-polarized collimated radiation consists, according toFIG. 1, of the polarizer 3, rotator 4, and analyzer 5. For polarizedinput radiation the device consists only of the rotator 4, and analyzer5. The correct setting of the rotation angle of the analyzer can bedetected electronically or visually.

Example No.3

[0016] This method also allows for a simple determination of thespectral half-width of lights sources emitting radiation centered at thewavelength λ₀. For this type of radiation, it is no longer possible tofind the position of the analyzer, when the passing intensity equalszero. However, the analog angle φ_(λo) exists for whichS(φ_(λo))=S_(min). This means that for the given wavelength, there is arotation angle of the analyzer, for which the intensity of the analyzedpassing light is minimal. After turning the analyzer by 90°, it meansfor angle φ_(λo)+π/2 or φ_(λo)−π/2, it is possible to get reading of themaximum intensity S_(max). Because of the non-zero dispersion of theoptical rotation of the rotator, the value S_(min)/S_(max), according tothe above formula, is directly related to the spectral half-width of theanalyzed light. Different angles or the analyzer rotation, for which theoutput intensity is minimal, correspond to different wavelengths λ₀.This procedure can be used for determination of the spectral half-widthof the light of pulsed femtosecond lasers. Because in these lasers thespectral half-width of the emitted radiation is in direct relation withthe duration of the light pulse, this method allows for a measurement ofthe duration of the ultra-short light pulses.

[0017] The device for the measurement of the radiation half-widthconsists, according to FIG. 1, from the polarizer 3, rotator 4, and theanalyzer 5. For the polarized input radiation, the device consists onlyof the rotator 4, and the analyzer 5. Setting of the analyzer angle canbe detected electronically or visually.

[0018] Industrial Application

[0019] The invention can be used everywhere where it is necessary toanalyze spectral composition of light. The application is ideally suitedfor situations where it is necessary to generate a color-contrast withmonochrome CCD cameras. The new method can also be used for preciseabsolute measurement of the wavelength of a monochromatic radiation andfor identification of the spectral half-width of the light sources witha band-shaped spectral profile. For the femtosecond lasers, it ispossible to determine the pulse width from the spectral half-width.

1. A method for the identification of the spectral characteristics oflight, where the analyzed beam of radiation with a spectrum I(λ) fromthe wavelengths interval (λ₁,λ₂) is converted to the linearly polarizedparallel beam of radiation and this collimated polarized radiation issubjected to optical rotation in the optically active environment withdispersion of the optical rotation. This results in a rotation of thepolarization plane of the individual spectral components of theradiation by the angle θ(λ), depending on their wavelength λ. Such newlyformed parallel beam is again polarized in the plane that forms angleφ=φ₁ with the polarization plane of the linearly polarized analyzedradiation. Then the output intensity of the analyzed beam is measured.Then the angle φ is changed and the output intensity of theanalyzed-radiation is measured again. This procedure is repeated for aset of angles φ from the interval (φ₁, φ₂) and the function dependenceS(φ) of the output intensity of the analyzed radiation on the angle φ isobtained: S(ϕ) = ∫_(λ₁)^(λ₂)I(λ)cos²[θ(λ) − ϕ]λ.

From this dependence, the spectral characteristics of the analyzed lightI(λ) is determined:
 2. A method according to the claim 1 , where duringthe process of determination of the spectral characteristics of thepoint light source, the collimated polarized radiation passes through anoptically active environment that exhibits the same dispersion of theoptical rotation in the entire crossection of the analyzed beam. Theoutput intensity is measured with a single-channel detector.
 3. A methodaccording to claim 1 where during the process of determination of thespectral characteristics of the planar source of light, the collimatedpolarized radiation passes through an optically active environment thatexhibits the same dispersion of the optical rotation in the entirecrossection of the analyzed beam. The output intensity is measured witha multi-channel detector.
 4. A method according to the claim 1 -3, wherethe minimum and maximum of the output intensity of light is measured.From these values, the wavelength of the spectral maximum and thespectral half-width of the spectral band of the analyzed radiation isdetermined by comparison of the values with the experimentalcalibration, or with the calibration determined from the formulapresented in the claim 1 .
 5. A device for performing methods accordingto the claims 1-4, consisting of a collimating optical system and asingle or multi-channel detector that has the first polarizer (3),rotator (4) and second polarizer (5), placed between the optical system(2) and the detector (7).